Even if these gods succeed in fusing completely with the two pathways, it wouldn't change the fact that Legion possessed the authority of the pathway of the undying, which made them incredibly difficult or even impossible to kill. So Legion was still able to survive and thrive despite the exposure of their capabilities.
Legion had the power to slowly accumulate the authorities they needed to create their Supreme authority. Unfortunately, every god that they should be able to kill chose to hide and disappear from the world, so Legion wasn't able to find them.
The Legion of the past didn't know why things became so bad for them, so they didn't know how to solve their problem. This effectively cut off the path of Legion and made it nearly impossible for them to create their 13 authority Supreme authority, reconnect with their origin, and become an Absolute.
